1. Field of the Invention
The present invention relates to a hot-dip galvanized steel sheet to be used as a corrosion preventive steel sheet for automobiles, or the like. More particularly, the present invention relates to a hot-dip galvanized steel sheet which has ensured both of good plating adhesion and good formability. Incidentally, the term xe2x80x9chot-dip galvanized steel sheetxe2x80x9d used in the present invention embraces not only ordinary ones carrying the zinc layer in the as-plated state but also so-called hot-dip galvannealed steel sheets which have undergone a heat treatment for alloying after galvanizing (to form the zinc layer on the basis material).
2. Description of the Related Art
There is an increasing demand for improvement in fuel consumption rate as a part of the policy for preventing the global warming due to CO2 discharge. To this end, a new target for improved fuel consumption has been set up and a new tax system has been introduced in favor of cars with improved fuel economy. One effective way of improving fuel consumption rate is by reduction of body weight. Achieving this object requires the raw material to have higher tensile strength than before. This is also the case with hot-dip galvanized steel sheets. For hot-dip galvanized steel sheets to have both high tensile strength and good formability, the basis material (steel sheet) should be incorporated with such elements as C, Si, and Mn.
Incidentally, for a hot-dip galvanized steel sheet, hot-dip galvanizing may be preceded by Fe electroplating (below, simply referred to as Fe plating) as preliminary plating (pre-plating), and then a step of annealing in a reducing atmosphere, with the aim of reducing the effect on the basis steel sheet, and the like. However, it has been observed as follows. If the steel sheet contains Si and Mn as chemical components for achieving higher tensile strength, these chemical components diffuse and concentrate onto the Fe plated layer surface due to heating during annealing. Accordingly, respective oxides or complex oxides (below, both the oxides will be simply referred to as oxides) of Si and Mn which are easily oxidizable elements, are yielded on the Fe plated layer surface due to the oxygen inevitably occurring in the atmosphere, although it is the reducing atmosphere. Unfavorably, when the hot-dip galvanized steel sheet is formed, peeling or the like occurs in the vicinity of the interface between the oxide layer and the hot-dip galvanized zinc layer (below, simply referred to as a Zn plated layer) due to the inferior adhesion between the oxide layer and the Zn plated layer.
Therefore, for manufacturing a hot-dip galvanized high tensile strength steel sheet, it is essential that the layer of Si and Mn oxides as described above should not be formed on the Fe plated layer surface. Various studies have been conducted from such a viewpoint. For example, Japanese Patent Publication No. 2618308 discloses the following method. In the method, a 0.001 to 1 xcexcm-thick mixed layer of Fe2SiO4 and SiO2 is formed at the lower part of the Fe plated layer to inhibit formation of Fe and Si oxide film detrimental to the platability on the Fe plated layer surface.
However, the inventors"" investigation has revealed that the foregoing method improves the adhesion of the Zn plated layer with reliability, but there is still room for improvement in terms of formability.
Under such circumstances, the present invention has been completed. It is therefore an object of the present invention to provide a hot-dip galvanized steel sheet having good formability by improving the adhesion between the basis steel sheet surface and a hot-dip galvanized zinc layer in a hot-dip galvanized high tensile strength steel sheet containing Si and/or Mn.
A hot-dip galvanized steel sheet of the present invention, which has attained the foregoing object, is composed of: a basis steel sheet containing Si in an amount of 0.05 to 2.5% and Mn in an amount of 0.2 to 3%, by mass; a Fe plated layer formed on the basis steel sheet; and a hot-dip galvanized zinc layer formed on the surface layer of the basis steel sheet via the Fe plated layer. The hot-dip galvanized steel sheet is characterized in that the oxides containing Si and/or Mn are discontinuously dispersed in the vicinity of the interface between the basis steel sheet and the Fe plated layer. By positively forming the oxides of Si and/or Mn in the vicinity of the interface between the basis steel sheet surface and the Fe plated layer, it is possible to reduce the amount of Si and/or Mn which may diffuse and concentrate onto the Fe plated layer surface during annealing after Fe plating. As a result, the adhesion with the hot-dip galvanized zinc layer is improved. Further, if the hard oxides are formed in laminar structure, peeling occurs at the interface between the oxide layer and the adjacent layers. For this reason, the formability is improved by discontinuously dispersing the oxides therein.
The state in which the oxides of Si and/or Mn are discontinuously dispersed denotes the following state. Namely, upon observation of how the oxides containing Si and/or Mn occur in a region of dimensions of 1 xcexcm or more by 1 xcexcm or more in a perpendicular cross section including the vicinity of the interface between the basis steel sheet and the Fe plated layer under a transmission electron microscope at a magnification of 50000, even if a line segment with a length of 600 nm is drawn at any position in the field of vision of observation, the length of the portion in which the line segment and the oxides containing Si and/or Mn overlap with each other does not exceed 480 nm. Further, if the preferred embodiments of the present invention are prescribed in terms of the size of each oxide containing Si and/or Mn, the major axis length of the oxide is preferably 400 nm or less.
Incidentally, as the oxides containing Si and/or Mn, SiO2 and Mn2SiO4 are conceivable, but other elements may be contained therein. Therefore, in the present invention, the oxides denote the moieties in which Si and/or Mn and O are detected at the same time upon EDS (energy-dispersive X-ray spectrometer) analysis in observation under a transmission electron microscope.
It is preferable that the basis steel sheet in the vicinity of the oxides containing Si and/or Mn contains Si and/or Mn in the form of solid solution in an amount of 80 mass % or less of the amount of Si and/or Mn in the composition of the basis steel sheet. Namely, the oxides are formed as a result of enrichment of Si and/or Mn in the form of solid solution in the peripheral basis steel sheet. Therefore, in the basis steel sheet in the vicinity of the oxides, the amount of Si and Mn in the form of solid solution is reduced. From such a viewpoint, the amount of Si and/or Mn in the form of solid solution has been prescribed as described above.